Fibers processed by intramolecular chemical crosslinking are known to have better fluid absorption properties and are indispensable for production of various living supplies, medical supplies, and the like. A crosslinking agent is commonly used for production of crosslinked fibers (also referred to as treated fibers), and various compounds are known as crosslinking agents for fibers (also referred to as fiber crosslinking agents). For example, Patent Literature 1 discloses individualized and chemically crosslinked cellulose fibers that contain cellulose fibers processed by intrafiber crosslinking with use of a polymer polycarboxylic acid crosslinking agent, the polymer polycarboxylic acid crosslinking agent having a molecular weight from about 500 to about 40000. In one embodiment disclosed, such a polymer polycarboxylic acid crosslinking agent is selected from the group consisting of polyacrylic acid polymers, polymaleic acid polymers, acrylic acid copolymers, maleic acid copolymers, and mixtures of these. Patent Literature 2 discloses cellulose fibers having intrafiber crosslinks formed with a phosphinate-containing telomer of acrylic acid that has a penetration factor of at least 65 and a Tgd from about 70° C. to about 105° C.